Object reference not set to an instance of an object in C# when use tree

Question

I use a function that save string into a string array but visual 2012 show this error Object reference not set to an instance of an object.

this is my code :

 public void ShowLinkList(ref TreeNode t, ref string[] Data, ref int i)
   {
       string str;
       if (t!= null)
       {
           ShowLinkList(ref t.LeftChild, ref Data, ref i);
           Data[i] = CreatStr(t.Parent.Word) + "," + CreatStr(t.LeftChild.Word )+ "," + CreatStr(t.RightChild.Word) + "," + CreatStr(t.Word);
           i++;
           ShowLinkList(ref t.RightChild, ref Data, ref i);
       }
   }
   public string CreatStr(string str)
   {
       if (str == "")
       {
           return "__";
       }
       return str;
   }

when t is null,the "if(t != null)" not allowed compiler to debug ,the CreatStr(string) convert a null string to "__" in output(windows form C#) this metode (ShowLinkList) saving t.parent.word & t.leftchild.word & t.rightchild.word & t.word in a string array please help me.thank you

On what line? my guess is `t.Parent/LeftChild/RightChild` are null... and what does this have to do with html/css? — Sayse, May 29 '14 at 06:32
Please debug the code and find exact point where this exception is coming, please put the code in try catch. — Incredible, May 29 '14 at 06:32
Your CreatStr method does not convert a null string, it converts an empty string. Try changing the if to if(String.IsNullOrEmpty(str)) — Mike Norgate, May 29 '14 at 06:41
http://stackoverflow.com/questions/4660142/what-is-a-nullreferenceexception-and-how-do-i-fix-it — Soner Gönül, May 29 '14 at 06:42
Data[i] = CreatStr(t.Parent.Word) + "," + CreatStr(t.LeftChild.Word )+ "," + CreatStr(t.RightChild.Word) + "," + CreatStr(t.Word); — Programmer, May 29 '14 at 06:44

Sayse · Accepted Answer · 2014-05-29T06:52:02.420

0

First, "" is equal to string.Empty and not equal to null

You have no error handling for any of the properties of t so if any of these are null then it won't have a clue what Word is, I'd suggest at the very least creating the following method on TreeNode

public bool HasValidBranches()
{
    return Parent != null && LeftChild != null && RightChild != null;
}

then add this to your error handling that is already present and feel free to modify to suit your needs

if(t != null && t.HasValidBranches())
....

Note: You could always just add the extra null checks directly to the current code.

edited May 29 '14 at 06:52

answered May 29 '14 at 06:45

Sayse

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I try to use this ..but not debug!!! – Programmer May 29 '14 at 06:54
I try to save parent and rightchild and leftchild of a word in a text file – Programmer May 29 '14 at 07:00
and maybe compiler not allowed to saving a null value to a string array,because maybe a parent node is null and i try change null to "-" to print at output – Programmer May 29 '14 at 07:03
This has nothing to do with trying to save a null value to a string array, you can't concatenate a string to null, and plus I don't think it even gets that far, `Word` is not a property of `null` – Sayse May 29 '14 at 07:04

Object reference not set to an instance of an object in C# when use tree

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